1. Field of the Invention
The present invention relates to a dielectric filter for use in a high frequency band, a dielectric duplexer, and a communication apparatus using the dielectric filter or the dielectric duplexer.
2. Description of the Related Art
FIGS. 10A, 10B, and 10C show the configuration of a dielectric filter using a dielectric block, mainly for use in a microwave band. FIG. 10B is a front view showing the dielectric filter which is made to stand. FIG. 10A is a plan view thereof. FIG. 10C is a bottom view thereof. FIG. 10D is a left side view thereof. FIG. 10E is a right side view. In FIG. 10, a dielectric block 1 has inner conductor formed holes 2a, 2b, and 2c formed inside thereof, and inner conductors 5a, 5b, and 5c are formed on the inner walls of the holes 2a, 2b, and 2c. On the outer surface of the dielectric block 1, an outer conductor 3 is formed, and terminal electrodes 6 and 7 are provided in predetermined positions so as to be insulated from the outer conductor 3. The terminal electrode 6 is capacitively coupled to the inner conductors 5a, and the terminal electrode 7 to the inner conductor 5c. Like this, a dielectric filter composed of three-stage resonators and having a band-pass characteristic is formed.
However, in the dielectric filter as shown in FIGS. 10A to 10E, the terminal electrodes 6 and 7 input or output signals unbalanced with respect to the earth potential (the potential of the outer conductor) as a reference potential. Accordingly, in order to give signals to an amplifier circuit of a balanced input type, for example, it is needed that unbalanced signals are converted to balanced signals by means of a balun (unbalanced-to-balanced transformer). As a result, the area on a circuit substrate, occupied by a filter circuit part, is increased. This is one of the causes which make it difficult to downsize the dielectric filter.
To overcome the above described problems, preferred embodiments of the present invention provide a dielectric filter which can input or output balanced signals, not using a balun, a dielectric duplexer, and a communication apparatus using the dielectric filter or the dielectric duplexer.
One preferred embodiment of the present invention provides A dielectric filter, comprising: a single dielectric member; a conductor film disposed on the single dielectric member; a xcex/2 resonator provided in association with the single dielectric member, having both-ends open or short-circuited, and resonating substantially at a half wavelength of a predetermined frequency; a xcex/4 resonator provided in association with the single dielectric member, having one short-circuited end and the other open end, and resonating substantially at a quarter wavelength of the predetermined frequency; a first terminal connected to the xcex/4 resonator and functioning as an unbalanced terminal; and second terminals respectively connected to the two open ends of the xcex/2 resonator and functioning as balanced terminals.
According to the above described structure and arrangement, the transmission mode can be converted by use of the unbalanced terminal and the balanced terminals, and moreover, signals in a predetermined frequency band range can be transmitted or attenuated.
In the above described dielectric filter, the dielectric constant of the portion of the dielectric member where the xcex/2 resonator is located may be higher than that of the dielectric member where the xcex/4 resonator is located so that the resonator lengths of the xcex/2 resonator and the xcex/4 resonator are substantially equal to each other.
According to the above described structure and arrangement, when the xcex/2 resonator and the xcex/4 resonator are formed by use of a dielectric block for example, no step is formed on the dielectric block. Therefore, no cracks are formed and also, the dielectric block can be easily clipped. This facilitates the production, and moreover, the handling of components when assembled in an electronic apparatus.
In the above described dielectric filter, the xcex/2resonator may be shaped so as to be doubled back substantially from the center thereof.
According to the above described structure and arrangement, even when the xcex/2 resonator and the xcex/4 resonator to be coupled to the xcex/2 resonator are formed on a single dielectric member with a uniform dielectric constant, the axial lengths of both of the resonators can be set to be equal to each other.
In the above described dielectric filter, at least one of the xcex/2 resonator and the xcex/4 resonator may comprise a micro-strip line or a strip line.
According to the above described structure and arrangement, a circuit provided with a filter, together with a circuit for inputting-outputting balanced signals and a circuit for inputting-outputting unbalanced signals, can be conveniently formed with no balun being provided.
In the above described dielectric filter, at least one of the xcex/2 resonator and the xcex/4 resonator may comprise a dielectric coaxial resonator made up of a dielectric block having a conductor film disposed thereon.
According to the above described structure and arrangement, the circuit provided with the filter, together with the circuit for inputting-outputting balanced signals and the circuit for inputting-outputting unbalanced signals, can be simply formed with no balun being provided, merely by mounting the dielectric filter on a circuit substrate, though the dielectric filter comprises a coaxial resonator.
Another preferred embodiment of the present invention provides a dielectric duplexer which includes a transmission filter and a reception filter each having the above-described configuration of the dielectric filter.
According to the above described structure and arrangement, the transmission mode can be converted and moreover, a transmission signal can be output, or a reception signal can be input by use of the unbalanced terminal and the balanced terminals.
Yet another preferred embodiment of the present invention provides a communication apparatus including the above described dielectric filter or the above described dielectric duplexer.
According to the above described structure and arrangement, a communication apparatus downsized and light in weight can be formed.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.